electrochemical behavior of supercapacitor electrodes ...2.2 electrochemical testing numerous...

Int. J. Electrochem. Sci., 12 (2017) 7287 – 7299, doi: 10.20964/2017.08.63

International Journal of

ELECTROCHEMICAL SCIENCE

www.electrochemsci.org

Electrochemical Behavior of Supercapacitor Electrodes Based

on Activated Carbon and Fly Ash

S. Martinović

1, M. Vlahović

1, E. Ponomaryova

2, I.V. Ryzhkov

2, M. Jovanović

3, I. Bušatlić

3, T. Volkov

Husović4, Z. Stević

5,*

1 University of Belgrade, Institute of Chemistry, Technology and Metallurgy, Belgrade, Serbia

2 Prydniprovsk State Academy of Civil Engineering and Architecture, Dnipropetrovsk, Ukraine

3 University of Zenica, Faculty of Metallurgy and Material Science, Zenica, Bosnia and Herzegovina

4 University of Belgrade, Faculty of Technology and Metallurgy, Belgrade, Serbia

5 University of Belgrade, Technical Faculty in Bor, Bor, Serbia

*E-mail: [email protected]

Received: 19 January 2017 / Accepted: 8 June 2017 / Published: 12 July 2017

The possibility of applying fly ash from power plants as a binder in supercapacitor electrodes based on

activated carbon was investigated in this research. Based on the mechanical and electrical properties of

the electrodes, the optimal ratio between fly ash and AC was determined. Supercapacitor electrodes

were prepared in two ways: by pressing and by laser solidification. The preparation method

significantly affected physical properties of the electrodes as well as the electrochemical behavior in

supercapacitor setup. The electrodes were electrochemically tested by galvanostatic and potentiostatic

methods and cyclic voltammetry. In order to improve the estimation of supercapacitor parameters,

mathematical model that perfectly describes the behavior of investigated electrodes in aqueous

solution of sodium nitrate was developed. The best results were obtained with laser-solidified electrode

in 1M aqueous solution of NaNO3. Specific capacitance of 105 F/g, serial resistance of 0.57 Ω and

self-discharge resistance of 95 Ω were achieved. Stability at high number of cycles proved to be very

good. After 2000 cycles of CV at scan rate of 100 mV/s, specific capacitance fell by only 4.6 %.

Keywords: supercapacitors; activated carbon; fly ash; laser solidification; NaNO3

1. INTRODUCTION

Storing energy in electrochemical double-layer capacitors (EDLC) is a result of charge

separation at the interface between the electrode as electronic conductor and the electrolyte as ionic

conductor of electricity. Capacitance that occurs at that interface is called a double-layer capacitance

[1,2].

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Int. J. Electrochem. Sci., Vol. 12, 2017

7288

Due to its high specific surface, activated carbon is one of the standard materials for

electrochemical double-layer capacitors. Electrochemical processes that occur at electrochemical

double-layer capacitor can be described as follows [3]:

Positive electrode:

eAEsAEs //

Discharge

Charge

(1)

Negative electrode:

CEseCEs //Discharge

Charge

(2)

Overall reaction:

CEAEACEE ssss ////Discharge

Charge

(3)

where Es is carbon electrode surface area, // is double layer with both sides for charge

accumulation, C+ and A

- are cationic and anionic species present in the electrolyte. Based on the above

reactions, it can be concluded that during charging, the electrons are pushed from the positive to the

negative electrode by an external source, while at the same time the positive and negative ions separate

and move towards the surfaces of electrodes. During discharge through a consumer the electrons move

from the negative to the positive electrode, and ions release from the electrodes surfaces and return to

the electrolyte. According to the overall reaction, salt (C+A

-) from the electrolyte is consumed during

the charging, so that the electrolyte can be considered as active material. Charge density at the

electrode-electrolyte interface changes during charging and discharging as well as concentration and

conductivity of the electrolyte [4].

Supercapacitors have higher specific power than batteries and for this reason they are combined

with batteries in electric devices. Other advantages are related to the longer service life, work in a

wider temperature range and fast charge-discharge. Further investigations are directed towards

increasing their specific energy [5].

The main factors that dictate choosing carbon for numerous electrochemical applications are its

availability, low cost, easy processing, as well as different forms (powder, nano fibers and tubes, foam,

fabric, composites) [6] and adjustable porosity [7-11]. Carbon electrodes are well polarisable,

chemically stable in various solutions (acidic, basic) and in a wide temperature range. The electrode

material has to be electrochemically inert in the potentials operating range, which is limited by the

electrolyte decomposition potential [12].

In aqueous solutions supercapacitor voltage cannot exceed 1 V, so it is clear that organic

solutions by giving greater energy have significant advantages compared to the water solutions.

Unfortunately, organic electrolytes mainly have lower conductivity, which reduces the specific power.

In order to improve the electrical and mechanical properties, supercapacitor electrodes are often

made of composite based on carbon and suitable binders (for example poly(vinylidene difluoride)

(PVDF) or poly(tetrafluoroethylene) (PTFE)) [13-16]. The possibility of applying ash for this purpose

was also investigated [17,18].


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Fly ash is formed as waste product in thermal power plants (TPP) fired by coal. The term "fly

ash" implies a residue after combustion (for energy purposes) of ground coal, which distinguishes it

from all other ashes. In addition, fly ash has specific characteristics significantly different from other

ashes and other industrial mineral wastes. The chemical composition of fly ash varies depending on the

kind of used fuel and combustion technology while the presence of carbon, whose content can vary

from 2 to 10 wt.%, considerably affects the electrical conductivity of the material [19].

The aim of this study was to improve the process of producing supercapacitor electrodes based

on active carbon with the addition of fly ash from the power plant as a binder. The composite solidified

using laser was compared with the composite obtained by conventional pressing.

2. EXPERIMENTAL

2.1 Electrode preparation

Preliminary experiments were performed with various ratios of fly ash (FA) and activated

carbon (AC) (5, 10, 15 and 20 wt.% FA) and based on mechanical and electrical properties of the

electrodes it was determined that optimal composition is 10 wt.% of FA and 90 wt.% of AC. Activated

carbon (Aktivkohle, MERCK) was used as basic active material and FA from the Thermal Power Plant

”Nikola Tesla A”, Obrenovac, Serbia was used as a binder.

Main physical properties of used FA are given in Table 1 and Figure 1.

Table 1. Physical properties of FA from TPP ”Nikola Tesla A”

Specific surface area

(cm2/g)

(g/cm3) Ωm) LOI (%)

4487.52 0.56 4.3ˑ107 3.75

Figure 1. Particle size distribution of FA from TPP ”Nikola Tesla A”.


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AC/FA electrodes were prepared using two methods. Method 1 consisted of manual mixing of

the components (10 wt.% of FA and 90 wt.% of AC), followed by pressing at different pressures along

with gold-plated brass surface. Optimal pressure was 20 kN/cm2, as a compromise between mechanical

stability and porosity of the electrode. On the other side of gold-plated substrate copper conductor was

soldered and at the end everything except active surface was plastered in the Epofix resin (Epoxan,

Čačak, Serbia). The prepared electrode was marked with E1. The construction of the electrode is

illustrated in Figure 2, and Figure 3 shows microphotograph of the electrode active surface.

Figure 2. Construction of the electrode.

Figure 3. Microphotograph of electrode active surface E1.

Method 2 is the same as method 1 in all preparation stages, except that laser solidification is

applied instead of pressing. Continuous unpolarised semiconductor lasers PGL-F series, CNI (power

2W, wavelength 445 nm) was used for that purpose. The mixture AC/FA was scanned by laser at a rate

of 1mm/min; thus compact electrode was obtained, and high porosity remained. Besides reinforcing

effect, specific resistance decrease of ash was noticed (4.5ˑ106 m). After sealing into the epoxy mass,


7291

the active electrode surface was grinded and polished. It was marked with E2. Figure 4 presents

microphotograph of the electrode active surface.

Figure 4. Microphotograph of electrode active surface E2.

2.2 Electrochemical testing

Numerous standard testing methods for electrochemical systems can be applied on

supercapacitors, with certain modifications [1,20,21]. In this research the method of cyclic

voltammetry, galvanostatic method of charging and discharging and potentiostatic method were used.

In order to monitor the process on a single electrode (electrode-electrolyte interface), three-

electrode electrochemical cell was used. One of the tested electrodes (E1 or E2) was connected as a

working electrode. A saturated calomel electrode (SCE) was used as a reference electrode and a

platinum mesh as a counter electrode. Operating electrolyte was 1M aqueous solution of sodium nitrate

(NaNO3).

The electrochemical cell was connected to a standard galvanostat- potentiostat Interface 1000

from GAMRY Instruments. The system includes software for all necessary electrochemical testing

methods. All tests were performed at room temperature.

2.3 Mathematical modeling of electrode behavior in NaNO3 aqueous solution

A simplified analog circuit (Figure 5), which describes well enough behavior of carbon

materials in aqueous solutions, was selected starting from the complex model [1,20].

Figure 5. Analog electrical circuit.


7292

The resistance Rs physically corresponds to the resistance of the electrolyte and electrode

material with ohm range value. The resistance R (with tens of ohms range value) is related to the slow

adsorption and diffusion processes. Capacitance C (farad range value) is an integrated capacitance. RL,

with kiloohm range value, is the resistance of self-discharge, therefore it is reciprocally connected with

the leakage current.

For adopted analog circuit and applied electrochemical test methods for supercapacitors, in

other words for the corresponding excitation, circuit response was analytically determined.

Experimentally obtained response diagrams of voltage or current show the validity of the model, while

circuit parameters were calculated by entering characteristic read values into a mathematical model.

2.3.1 Cyclic voltammetry

Cyclic voltammetry is one of the standard electrochemical methods [1,20]. The analysis of its

application for supercapacitors, that is for model presented in Figure 5, is presented in this research.

Excitation overpotential can be analytically presented as:

)(t tt

Em

1

for ascending part (charging phase) (4)

)(t tt

EE m

m

1

2 for descending part (discharging phase) (5)

With real assumption that RS+R << RL, analysis of circuits leads to simplified expressions for

the current in the time domain and in a quasi-stationary regime (after several cycles):

t

m

eCt

Eti 21)(

1

charging phase (6)

12)(1

tm

eCt

Eti discharging phase (7)

(8)

RS+R)C time constant of charging/discharging (9)

Time is measured from the beginning of each phase. Besides capacitance, equivalent serial

resistance RS + R is one of the most important parameters of supercapacitors. Usually it is represented

by the abbreviation ESR.


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Plateau current, that is maximum current will be:

Ct

EII m

1

minmax (10)

Integral capacitance C can be directly calculated by entering Imax read from the experimentally

obtained diagram into the Equation (10). However, in some cases, current plateau is not pronounced,

so that calculating C from the area A marked by the curve i- is more reliable:

2

m

1

2E

AtC (11)

Time constant is obatined from the intercept, and then the total series resistance (ESR) is

calculated from the Equation 9.

Self-discharge resistance value RL of good supercapacitors is high, but does not affect the shape

of the curve CV. Therefore, it cannot be determined by CV method. In the case of pronounced self-

discharge, a slightly rising straight line, whose slope is reciprocally linked with RL, will appear instead

of the current plateau.

2.3.2 Galvanostatic charging and discharging

For the assumed analog electrical circuit (Figure 5), the Equation for overpotential during

charging by constant current I is derived as follows:

t

LS eIRIRRt 1)()( (12)

where:

CRL (time constant) (13)

Typical shape of galvanostatic curve with characteristic data that can be used to calculate all

parameters of the equivalent electrical circuit, is shown in Figure 8.

If complete electrochemical curve is recorded (for the pulses duration greater than 4τ with the

aim to achieve stationary level of ηm, which is quite a long time), the procedure for obtaining the

circuit parameters would be as follows:

1. ηm = RL I (14)

is read from the diagram, where:

IR m

L

(15)

2. η0 = (RS + R) I = ESR I

is read, so

IESR 0

(16)

3. From the read time constant τ and (13), capacitance is calculated:

LRC

(17)


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Since the time constant τ is very large in this case, the current direction I often changes after the

charging to some extent (less than 1 V for the aqueous solutions) and therefore supercapacitor would

discharged by exponential law as well.

Overpotential drop 2 I ESR occurs at the time of changing the current direction, from where

ESR can be determined more precisely. In this case, the capacitance C is calculated from the slope of

initial (linear) part of charging curve:

C

I

dt

d

(18)

2.3.3 Potentiostatic method

If the excitation of electric circuit from the Figure 5 is long overpotential pulse, response of the

system (current in this case) will be:

L

t

L IeIIti

0)( (19)

where:

RR

EI

S 0 - initial charging current (20)

LS

LRR

EI

- final charging current (leakage current) (21)

CRRS )( - time constant (22)

Based on the experimentally obtained current diagram, after the reading of I0, IL and τ,

parameters of the circuit can be determined:

L

LI

ER (22)

0I

EESR (23)

ESRC

(24)

This method has the great advantage comapred to others. Resistance RL can be determined

most reliable from the clearly perceptible horizontal part of the curve. Also, RL can be accurately

determined by the method of self-discharge. However, the time constant CRL is much greater in

that case, so experiment lasts long time.

2.3.4 Energy density, power density and leackage time constant

Some important characteristics of supercapacitors can be determined using the obtained

parameters of equivalent electrical circuit (ESR, RL and C). These are the energy density (WS), the

power density (PS) and the leackage time constant ():

2

2

1mS CEW (25)


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ESR

EP m

S

2

(26)

LL CR (27)

3. RESULTS AND DISCUSSION

A series of experiments were done while the representative results obtained by different

methods of electrochemical tests are presented here.

3.1 Cyclic voltammetry

Cyclic voltammograms for electrodes E1 and E2 in 1M aqueous solution of sodium nitrate

(NaNO3) at the overpotential changes speed of 2 mV/s are shown in Figure 6. For comparison purpose,

results from the Ref [13] are shown in Figure 10 as well.

Figure 6. Cyclic voltammograms of tested electrodes (E1 and E2) and AC-PTFE composite.

From the diagram shape, it is obvious that the best electrode is E2 (composite AC/FA solidified

by laser). The largest area of loop indicates greater capacitance, while the existence of power plateau

means high self-discharge resistance RL (not case in the AC/PTFE sample [13]). Electrode E1 (pressed

composite AC/FA) is inferior to the samples E2 and [13] in terms of capacitance and ESR, but (similar

to E2) is excellent in terms of self-discharge.

Quantitatively expressed results obtained by the Equations (6-11) are given in Table 2.


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Table 2. Parameters of supercapacitors determined by the method of cyclic voltammetry

Sample

Parameter

E1 E2 AC/PTFE [13]

ESR, Ω 0.798 0.569 0.46

C, F/g 92.1 104.9 99.2

It is obvious that the laser-solidified electrode E2 showed the best results. Stability testing of E2

exhibited that capacitance dropped for 4.61 % after 2000 CV cycles at a scan rate of 100 mV/s.

In order to verify the model and obtained results, galvanostatic tests were carried out.

Experimental diagrams for electrodes E1 and E2 in 1M aqueous solution of sodium nitrate (NaNO3) at

the amperage of 100 mA/g and pulse duration of 800 s are shown in Figure 7.

Figure 7. Galvanostatic diagrams for electrodes E1 and E2.

Parameters of supercapacitor ESR and C were calculated by reading the characteristic values

from the experimental diagrams and applying the Equations 16 and 18, Table 3.

Table 3. Parameters of supercapacitors determined by the galvanostatic method

Sample

Parameter

E1 E2

ESR, Ω 0.795 0.567

C, F/g 92.2 105.1

Due to the accuracy of reading, more accurate ESR value is obtained by this method, while the

value of C is more accurate using CV method. However, the results are in very good agreement.


7297

3.3 Potentiostatic method

With the aim of more accurate determination of important supercapacitor parameters, self-

discharge resistance RL, potentiostatic tests were done.

Time diagram of responsive current for electrodes E1 and E2 in 1 M aqueous solution of sodium

nitrate (NaNO3), at potentiostatic pulse with intensity of 100 mV and duration of 500 s is shown in

Figure 8.

Figure 8. Potentiostatic diagrams for electrodes E1 and E2.

The intercepts, height of the horizontal (final) part of the curve and slope of the initial part of

the curve are clearly visible in the diagram, so all important parameters of supercapacitor can be

obtained by applying the Equations 22-24, Table 4.

Table 4. Parameters of supercapacitors determined by the potentiostatic method

Sample

Parameter

E1 E2

ESR, Ω 0.794 0.566

RL, Ω 96.8 95.1

C, F/g 91.8 104.8

Using this method and relatively short experiment, quite accurate values of self-discharge

resistance RL can be obtained, while other methods are used only for qualitative analysis. Thereby, the

value of ESR is determined very precisely by this method, while the CV method is inviolable for

determining the capacitance C.


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Comparing the results from the Table (2-4), it is obvious that ESR and C are in very good

agreement so the proposed model corresponds to the observed system. Also, obtained values are

comparable with results of the other studies engaged with AC in aqueous solutions [13-15].

3.4 Energy density, power density and leackage time constant

The other important parameters, WS, PS and τL, are calculated using the basic parameters of

super capacitors (ESR, RL and C) and based on the Equations 25-27 and shown in Table 5.

Table 5. Energy density, power density and leackage time constant

Sample

Parameter

E1 E2

WS, Wh/kg 8.196 9.324

PS, W/kg 806 1130

L, s 8915 9976

Obtained values are comparable with results of the other studies engaged with AC in aqueous

solutions [13,14]. Thereby, significant advantages show the laser-solidified electrode E2.

4. CONCLUSION

Testing of AC/FA composite in aqueous solution of sodium nitrate was carried out in order to

optimize electrochemical system for obtaining the best possible propeties (as much as possible specific

capacitance, the lowest series and the highest parallel resistances). Simplified equivalent electrical

circuit that well describes the behavior of observed electrochemical system was selected. Mathematical

model that enabled determination of the system parameters was developed based on the electrical

circuit and using the experimental results at different excitations. Optimization of the electrode

synthesis was also realized. Besides the conventional pressing, the possibility of solidification by laser

was also tested and it showed good results. Optimized electrodes (pressed and solidified by laser) were

examined by selected electrochemical methods (cyclic voltammetry, galvanostatic and potentiostatic

methods). Obtained results showed validity of the proposed model and the electrode material,

particularly AC/FA composite solidified by laser.

ACKNOWLEDGMENTS

This research has been financed by Ministry of Education, Science and Technological Development of

the Republic of Serbia as part of the projects TR 33007, III 45012 and ON 172060.

References

1. B.E.Conway, Electrochemical supercapacitors, Kluwer Academic/Plenum Publishers, (1999) New

York, USA.


7299

2. R. Kotz, M. Carlen, Electrochim Acta, 45 (2000) 2483.

3. J.P. Zheng, J. Huang and T.R. Jow, J Electrochem Soc, 144 (1997) 2026.

4. M. Rajčić-Vujasinović, Z. Stanković, Z. Stević, Elektrokhimiya, 35 (1999) 347.

5. C. Liu, Z. Yu, D. Neff, A. Zhamu and B.Z. Jang, Nano Lett, 10 (2010) 4863.

6. Z. Q. Gong, A. N. A. Sujari and S. Ab Ghani, Electrochim Acta, 65 (2012), 257.

7. K. Pokpas, S. Zbeda, N. Jahed, N. Mohamed, P. G. Baker, and E. I. Iwuoha, Int J Electrochem Sci,

vol. 9 (2014) 736.

8. B.K. Kim, S. Sy, A. Yu and J. Zhang, Electrochemical Supercapacitors for Energy Storage and

Conversion, John Wiley & Sons, Ltd., (2015).

9. E.Frackowiak and F.Béguin, Carbon, 39 (2001) 937.

10. W. Xing, S.Z. Qiao, R.G. Ding, F. Li, G.Q. Lu, Z.F.Yan and H.M.Cheng, Carbon, 44 (2006)

216.

11. Z. Wen, D. Lu, S. Li, J. Sun and S. Ji, Int J Electrochem Sci, 9 (2014)1.

12. A. González, E. Goikolea, J.A. Barrena and R. Mysyk, Renew Sust Energ Rev, 58 (2016) 1189.

13. Q. Abbas, D. Pajak, E. Frackowiak and F. Béguin, Electrochim Acta, 140 (2014) 132.

14. Z. Zhu, S. Tang, J. Yuan, X. Qin, Y. Deng, R. Qu and G.M. Haarberg, Int J Electrochem Sci, 11

(2016) 8270.

15. B. Gao, Y. Li, Y. Tian and L. Gai, Int. J. Electrochem. Sci., 12 (2017) 116

16. A. Ajina and D. Isa, J Sci & Technol, 18 (2010) 351.

17. L. Weinstein and R. Dash, Materials Today, 16 (2013) 356.

18. Y.M. Chen and H.Y. Zhang, Adv Mater Res, 150-151 (2011) 1560.

19. D. Jozić, Studija utjecaja letećeg pepela iz termoelektrane na fizikalno-kemijska svojstva i

ponašanje cementnog kompozita, doktorska disertacija, Sveučiliste u Splitu, Kemijsko-tehnološki

fakultet, Split (2007).

20. Z. Stevic, M. Rajcic-Vujasinovic and I. Radovanovic, Int J Electrochem Sci, 9 (2014) 7110.

21. Z. Stevic, M. Rajcic-Vujasinovic, I. Radovanovic, V. Nikolic, Int J Electrochem Sci, 10 (2015)

6020.

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electrochemical behavior of supercapacitor electrodes ...2.2 electrochemical testing numerous...

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